Traditionally, in static conformal radiation therapies the dose rate was relatively unimportant, and accelerator designs tended to work with a fixed Pulse Repetition Frequency (PRF) at which the RF source was pulsed in order to create radiation pulses. To vary the rate at which dose was imparted to the patient, the base frequency could be halved, repeatedly if necessary. Thus, the apparatus generated fixed dose rates at fixed divisors e.g. ½, ¼, . . . of the maximum output power, corresponding to like fractions of the base frequency.
This design approach was employed on accelerators because these systems were originally designed with power supplies that were susceptible to the AC effect of the mains voltage. In particular the High Voltage of the Modulator used to pulse the Magnetron would have a ripple at the frequency of the mains supply, and this ripple directly coupled through the Magnetron into the frequency of the RF and then to a ripple on the dose rate. This effect could be avoided by ensuring that the Modulator was always pulsed at a fixed position relative to the incoming AC supply. Hence, a 400 Hz pulse rate would be suitable for a 50 Hz supply. Modern accelerator systems typically use switched mode power supplies which are not susceptible to the mains AC and therefore are not constrained in this way.
This approach imposes limitations when the apparatus is used in modern Volumetric Modulated Arc Therapy (VMAT) therapies, such as we describe in our WO2007/124760. In these dynamic techniques, the dose rate is used as a variable in order to match the maximum speed of the moving machine axes and therefore create the specified dose, expressed as mu/mm or mu/.degree. Thus, when the optimising algorithm comes to calculate the ideal dose rate for a portion of the treatment, it has only a very ILO limited set of dose rates to choose from. This leads to situations where the optimising algorithm is forced to select a dose rate that can be almost half the optimal dose rate, because of the simple PRF divisors used. The end result of this is longer treatment times.
This limitation also causes step changes in other machine parameters, such as the gantry speeds and leaf speeds. This causes a reduction in delivery accuracy, because the servo systems must compensate for gross changes in demand.